Memory devices generally include volatile memory devices and non-volatile memory devices. Volatile memory devices generally include DRAM (dynamic random access memory) devices and SRAM (static random access memory) devices. Usually, volatile memory devices have the advantage of high access speed and the drawback of high power consumption because power supply must not be shut down to prevent loss of the data stored therein. Non-volatile memory devices have higher storage density per unit volume compared with volatile memory devices and lower power consumption because the data stored therein will not be lost even after power supply is shut down. The access speed of non-volatile memory devices is lower than most volatile memory devices but higher than most mechanical hard drives.
Most memory devices used in computer memory systems are volatile memory devices such as DRAM devices. Non-volatile memory devices are usually used in external storage devices, such as solid state disks and USB disks, accessed through I/O interfaces.